Gears can be classified by tooth shape, gear shape, tooth line shape, surface on which the tooth is located, and manufacturing method. The tooth profile of the gear includes the tooth profile curve, pressure angle, tooth height and displacement. Involute gears are relatively easy to manufacture, so involute gears account for the majority of gears in modern use, while cycloidal gears and circular arc gears are less used. In terms of pressure angle, the carrying capacity of small pressure angle gears is small; The large pressure angle gear, although the load carrying capacity is higher, but the load of the bearing becomes larger under the same transmission torque, so it is only used in special cases. The tooth height of the gear has been standardized, and the standard tooth height is generally used. The advantages of variable gears are many and have been widely used in all kinds of mechanical equipment. In addition, gears can also be divided into cylindrical gears, bevel gears, non-circular gears, racks, worm gears, and worm gears according to their shape; According to the shape of the tooth line, it is divided into spur gear, helical gear, herringbone gear and curved gear; According to the surface where the gear teeth are located, it is divided into external gears and internal gears; According to the manufacturing method, it can be divided into casting gear, cutting gear, rolling gear, sintered gear, etc.